WO2000051270A1

WO2000051270A1 - Digital optical transmitter with removable digital module

Info

Publication number: WO2000051270A1
Application number: PCT/US2000/004310
Authority: WO
Inventors: Forrest M. Farhan
Original assignee: Scientific-Atlanta, Inc.
Priority date: 1999-02-22
Filing date: 2000-02-18
Publication date: 2000-08-31

Abstract

An optical transmitter (810) receives electrical signals that are transmitted in a reverse direction and transmits optical signals, corresponding to the electrical signals, in the reverse direction. The optical transmitter (810) includes a reverse path circuit (860) for processing at least one analog electrical signal to generate therefrom a first digital bit stream and a digital module (890) that is detachably mounted within the optical transmitter (810) for processing additional analog electrical signals to generate therefrom a second digital bit stream. The digital module (890) includes a mechanical unit that can be mechanically and electrically inserted into and removed from the optical transmitter (810). The optical transmitter (810) further includes a multiplexer (850) coupled to the reverse path circuit (860) and the digital module (890) for receiving the first and second digital bit streams and providing a multiplexed output and a laser module (865) coupled to the multiplexer (850) for receiving the multiplexed output and generating therefrom a digital optical signal that is transmitted by the optical transmitter (810) in the reverse direction.

Description

DIGITAL OPTICAL TRANSMITTER WITH REMOVABLE DIGITAL MODULE

FIELD OF THE INVENTION

This invention relates generally to fiber optic communications, and more specifically to optical transmitters for use in fiber optic communications.

BACKGROUND OF THE INVENTION

Cable television systems typically include a headend section for receiving satellite signals and demodulating the signals to an intermediate frequency (IF) or baseband. The down converted signals are then modulated with radio frequency (RF) carriers and converted to an optical signal for transmission from the headend section over fiber optic cable. Optical transmitters are distributed throughout the cable system, such as at headends. for splitting and transmitting optical signals, and optical receivers are provided in remote locations within the distribution system for receiving the optical signals and converting them to radio frequency (RF) signals that are further transmitted along branches of the system over coaxial cable rather than fiber optic cable. Taps are situated along the coaxial cable to tap off downstream (also referred to as 'Outbound" or "forward") cable signals tαsubscribers of the system.

Various factors influence the ability to accurately transmit and receive optical signals within a cable television system. As the length of fiber optic cable within a system increases, for example, signal losses also increase. Furthermore, temperature fluctuations, which cause variation in the optical modulation index of the optical transmitter, can result in variation of the radio frequency (RF) output level of the optical receiver. Signal distortions can be caused b\ non-linearities in the laser and photodiode of the optical transmitter.

Although these problems can be mitigated by employing expensive techniques, e.g.. decreasing fiber lengths between optical nodes, such techniques may prohibitively increase costs to both subscribers and service providers. Thus, what is needed is a better way to provide reliable and accurate transmission of optical signals within a cable television system. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a cable television system in accordance with the present invention. FIG. 2 is an electrical block diagram of an optical transmitter included in the cable television system of FIG. 1 in accordance with the present invention.

FIG. 3 is an electrical block diagram of an optical receiver included in the cable television system of FIG. 1 in accordance with the present invention.

FIG. 4 is a block diagram of a cable television having multiple outputs to subscriber regions in accordance with the present invention.

FIG. 5 is an electrical block diagram of an optical transmitter included in the cable television system of FIG. 4 in accordance with the present invention.

FIG. 6 is an electrical block diagram of an optical receiver included in the cable television system of FIG. 4 in accordance with the present invention. FIG. 7 is a block diagram of a cable television system including a clock source in accordance with the present invention.

FIG. 8 is an electrical block diagram of an optical transmitter for receiving a clock signal from the clock source of FIG. 7 in accordance with the present invention.

FIG. 9 is an electrical block diagram of an optical transmitter having a laser module and a digital module in accordance with the present invention.

FIG. 10 is an electrical block diagram of the optical transmitter of FIG. 9 in which an additional digital module has been provided in accordance with the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 shows a communications system, such as a cable television system 100 having both forward and reverse paths, i.e., having the ability to communicate downstream in the forward direction and upstream in the reverse direction. The cable television system 100 includes a headend 105 for receiving satellite signals that are demodulated to baseband or an intermediate frequency (IF). The baseband signal is then converted to cable television signals that are routed throughout the system 100 to subscriber equipment 130, such as set top decoders, televisions, or computers, located in the residences or offices of system subscribers. The headend 105 can, for instance, convert the baseband signal to an optical signal that is transmitted over fiber optic cable 1 10, in which case a remotely located optical node 1 15 converts the optical signal to an electrical radio frequency (RF) signal for further transmission through the system 100 over coaxial cable 120. Taps 125 located along the cable 120 at various points in the distribution system split off portions of the RF signal for routing to subscriber equipment 130 coupled to subscriber drops provided at the taps 125.

The system 100, as mentioned, also has reverse transmission capability so that signals, such as data, video, or voice signals, generated by the subscriber equipment 130 can be provided back to the headend 105 for processing. The reverse signals travel through the taps 125 and any nodes 115 and other cable television equipment, e.g., reverse amplifiers, to the headend 105. In the configuration shown in FIG. 1, RF signals generated by the subscriber equipment 130 travel to the node 115, which converts the RF signals to optical signals for transmission over the fiber optic cable 1 10 to the headend 105.

In the system of FIG. 1, forward signals are typically provided in the frequency range of about 45 MHz to 900 MHz or higher, and reverse signals are typically provided in the frequency range of about 5 MHz to 40 MHz, although other frequency spectrums could alternatively be used. Referring to FIG. 2, a digital reverse transmitter 200 is provided for transmitting digital optical signals to the headend 105 in the reverse direction. The transmitter 200 can, for instance, be included within the optical node 1 15, although other locations within the cable television system 100 may also include the digital reverse transmitter 200 of the present invention. The transmitter 200 receives, at an input 202, an analog information signal that is representative of one or more reverse RF signals from the subscriber equipment 130. At its output 204, the transmitter 200 provides a digital optical signal that is generated in accordance with the analog information signal as well as an optional pilot tone that serves to provide a reference level during processing at the headend 105.

More specifically, the digital reverse transmitter 200 includes an analog-to-digital (AID) converter 205 for converting the analog input to a digital signal, i.e., a digital word comprising a particular number of bits, in a conventional manner. The resolution of the A/D converter 205, of course, is dependent upon transmitter design parameters. The transmitter 200 can also include a digital pilot tone generator 210 for providing a digital pilot tone in the form of a number of bits representative of a particular level and frequency. The digital pilot tone generator 210 could, for instance, include input switches by which the level and frequency could be varied. U.S. Patent No. 5,563,815 to Jones, the teachings of which are hereby incorporated by reference, shows a digital tone oscillator that could be used to implement the generator 210 included in the transmitter 200 of the present invention. A summer 215 receives the digital information signal from the A/D converter 205 and the digital pilot tone signal from the generator 210 and digitally adds the two signals by performing binary addition in a known manner. The summed signal is then coupled to a parallel-to-serial (P/S) converter, or a serializer 220, which receives the parallel inputs representative of the summed signal and converts the inputs into a serial bit stream. A laser diode 225 is then driven by an optical driver 222 to generate an optical signal in accordance with the serial bit stream. It will be appreciated that the serializer 220 can alternatively include a driver for driving the laser diode 225 and frame encoding circuitry for encoding the serialized digital signal into frames of data. FIG. 3 is a block diagram of an optical receiver 305 for receiving the digital optical signal transmitted by the optical transmitter 200. The receiver 305 can be, for instance, located in the headend 105, although other locations, such as any intervening hubs or nodes, may also employ the receiver 305. The receiver 305 includes a detector, such as a photodiode 310, for receiving the digital optical signal transmitted over the fiber optic cable 1 10 and generating therefrom a serial stream of electrical pulses in accordance with the optical signal. The output signals provided by the photodiode 310 are coupled to a serial-to-parallel (P/S) converter 315 for generating therefrom a set of parallel outputs corresponding to a digital word. The receiver 305 further includes a digital-to-analog (D/A) converter 320 for converting the signal provided at its digital input to an analog signal in a known manner. Thereafter, the analog signal is processed by a filter 325 to separate the pilot tone signal from the information signal. More specifically, the filter 325 preferably comprises a low pass filter that only passes the fundamental frequency component of the output of the D/A converter 320. As a result, the digital optical receiver 305 is able to provide at its output a reference signal, i.e., the pilot tone, and an analog signal that approximates the analog information signal initially provided to the optical transmitter 200. Furthermore, this can be done without encountering many of the problems that arise in prior art designs.

In conventional cable television systems, optical links in the reverse path use amplitude modulation to directly modulate a laser generating a reverse optical signal. As a result, RF output level of the optical receiver is directly dependent upon the optical modulation index (OMI), which in turn is directly related to the RF drive current, the laser threshold current, and the laser bias current of the laser located in the transmitter. Since the laser bias and threshold currents vary with temperature, which in turn causes temperature variations of the OMI, the RF output level of the optical receiver also varies with temperature. However, the laser within the transmitter 200 of the present invention is digitally modulated so that the RF level information is encoded according to a bit stream; as a result, variations in the OMI, the laser bias current, the laser threshold current, and the temperature do not affect RF output levels of the optical receiver 305.

Prior art optical transmission that uses AM modulation also result in a system in which the linearity of the received optical signal is directly dependent upon the linearity of the transmitting laser and the receiving photodiode. Therefore, non-linearities of those devices can greatly degrade the performance of the reverse path system. Additionally, the non-linear conversion processes of lasers and photodiodes in conventional systems vary with temperature, thus further degrading the performance. Conversely, the digital optical system, i.e., the digital optical transmitter 200 and the digital optical receiver 305, of the present invention only generates and resolves two amplitude levels rather than a continuum of levels. As a result, linearity requirements of the laser and photodiode are significantly reduced, which results in better performance and less expense.

Another problem associated with conventional cable television systems is that reverse pilot tones are seldom used due to the complications and costs. When such pilot tones are used, an additional oscillator, which is not digital, is generally located outside the transmitter and is susceptible to temperature variations. The oscillator signal is combined with the analog RF signal, and the combined signal is used to modulate the laser diode current to provide an optical output. Prior art pilot tones are used by an optical transmitter to ensure that there is always a minimum RF signal modulating the laser, thereby decreasing the spurious noise generated by the laser, and by an optical receiver for gain control purposes and for purposes of providing level calibration for received signals. However, since oscillator output level drifts with temperature, the RF output level of the optical receiver will also drift with temperature so that gain control is essentially useless. As mentioned above, use of the combined digital pilot tone and digital information signal according to the present invention solves the prior art temperature dependency problems and provides a flexibly configurable level calibration tone. At the same time, the digital pilot tone can be used by the transmitter 200 to modulate the laser even when no RF input is present.

Still another advantage of the digital optical transmitter 200 and receiver 305 of the present invention is that the cable system 100 can, without significant cost or performance penalties, employ an architecture in which fiber stretches deeper into the system 100. As a cable television signal travels along a fiber optic cable 1 10, the signal-to-noise ratio decreases as a result of laser noise, Rayleigh backscattering, photodiode shot noise, receiver amplifier noise, unmodulated Fabry-Perot sporadic noise, and post amplifier intrinsic noise. Conventionally, this problem is mitigated by driving the transmitter laser with more power and/or increasing the receive sensitivity of the receiver photodiode at great expense. However, this need not be done in a system 100 according to the present invention since the noise sources and corresponding signal degradation resulting from increased fiber lengths does not affect recovery of information to the same extent as in prior art systems.

Referring next to FIG. 4, a modified cable television system 400 is depicted. The system 400 includes a headend 105 for generating cable television signals that are split off to subscriber equipment 130 by taps 125. However, in the system 400, the optical node 415 splits off the downstream cable signal for transmission to multiple distribution systems 430, 435, or branches. Each branch typically provides service to subscribers located in different geographic regions. Upstream reverse signals provided by subscriber equipment 130 in the different branches 430, 435 is transmitted in the form of analog RF carrier signals to the optical node 415, which combines the signals for further upstream transmission in the form of an optical signal. According to the present invention, the upstream signals from the different branches 430, 435 can be converted to a digital optical signal in a manner that minimizes or eliminates many of the problems associated with prior art cable television systems.

FIG. 5 is an electrical block diagram of an optical transmitter 500 that can, in accordance with the present invention, be used to process multiple analog inputs. At input 502, the transmitter 500 receives a first analog input, such as from a first branch 430 of a cable television system 400, and, at input 503, the transmitter 500 receives a second analog input, such as from a second branch 435 of the system 400. First and second A/D converters 205, 505 respectively convert the received RF signals to digital information signals that are separately summed, by summers 215, 515, with the digital pilot tone. Each summed signal is then serialized by serializers 220, 520 to result in first and second serial bit streams that are representative of the first and second RF signals, respectively, as separately combined with the digital pilot tone.

According to the present invention, bits of the serial bit streams are interleaved by an interleaver 550 to form a single digital signal that modulates the laser diode 225, which is driver by optical driver 222. As a result, a single digital optical signal can be provided at the output 504 of the transmitter 500. Referring to FIG. 6, an optical receiver 605 for processing the digital optical signal generated by the transmitter 500 is shown. The receiver 605 includes a photodiode 310 for generating electrical pulses from the optical signal and a deinterleaver 650 for deinterleaving the signal comprising the electrical pulses. Once the deinterleaver 650 has separated the received signal into separate serial bit streams, the outputs are coupled to first and second S/P converters 315, 615, first and second D/A converters 320, 620, and first and second filters 325, 625 to recover replicas of the pilot tone and the RF signals that were provided to the transmitter 500.

It will be appreciated that the interleaver 550 and the deinterleaver 650 can be implemented using conventional components. Typically, the interleaver 550 could be a framing device capable of implementing a time-division-multiplexing (TDM) scheme with respect to the incoming bit streams. In such an implementation, a frame clock (not shown) would be coupled to the interleaver 550, and one frame would consist of a number of sub- frames equivalent to the number of incoming bit streams. A flag bit would likely be inserted into the frame for identifying the start of the frame. The deinterleaver 650 is capable of extracting the frame clock signal from the incoming information and recognizing the flag bits indicative of frame starts. Each bit would then be routed to its respective bit stream to recover the original signals.

Although only two input branches into the transmitter 500 and two processing paths through the transmitter 500 and the receiver 605 are shown, a plurality of paths can be provided depending upon the number of incoming analog signals to be processed by the transmitter 500. For example, if four RF signals are traveling in the reverse paths of four branches of a cable television system, the optical transmission system according to the present invention would individually convert each reverse signal to a digital signal, add it to the pilot tone, and serialize the combined signal. All serialized signals would then be combined by the interleaver 550 to generate a bit stream for modulating the laser diode 225 (FIG. 5). On the receiver end, the deinterleaver 650 would deinterleave the received digital optical signal to provide four serial signals that would be individually processed by S/P converters, D/A converters, and filters to provide four analog outputs as well as an approximation of the pilot tone.

In this manner, reverse signals of the same frequency can be conveniently sent to the headend 105 over the same return fiber 1 10. This is very important since cable television systems typically only allocate a small amount of bandwidth, e.g., 5-40 MHZ, for return path transmissions, which means that varying the frequency of each return path signal would not be practical.

Referring next to FIG. 7, a block diagram illustrates a cable television system 700 in which digital optical transmission and reception is clocked by a clock signal provided by a source 710 that preferably generates a sinusoidal signal of a particular frequency. The clock source 710 can be external, i.e., from outside the system 700, or internal to the cable television system 700. For example, the clock source 710 could be located in the headend 105 and coupled to nodes 715 for transmitting digital reverse optical signals and to any optical hub 705 for combining transmissions from the nodes 715 over a single fiber 1 10. When a cable television system, such as the system 700, is large enough to include a hub 705 and multiple branches, each including its own optical node 715, internal clock sources for each node could result in slight variations in the clock signals. If the clock signals were not synchronized precisely at the nodes 715, combining of the received signals at the hub 705 could cause erroneous reception of data at the headend 105. The use of the same clock signal for the hub 705 and nodes 715, on the other hand, ensures that data streams received by the hub 705 and retransmitted to the headend 105 are synchronized in time for accurate data transmission and reception.

FIG. 8 illustrates an embodiment of a digital optical transmitter 800 in which an external clock signal is provided. The digital optical transmitter 800 could reside in an optical node 715 or in the optical hub 705, as will be explained in greater detail below. Although the transmitter 800 is shown in FIG. 8 as receiving only four reverse electrical signals via input ports 720, the transmitter 800 can receive any number of reverse signals. Each input port 720 is coupled to a reverse path that includes an A/D converter 725 for converting the analog electrical signal to a digital electrical signal and a serializer 730 for converting the digital signal to a serial bit stream. Each serial bit stream within the transmitter 800 is provided to an interleaver 745 for interleaving the bits of data to generate a single bit stream that modulates the laser diode 750, which is driven by the optical driver 748 to provide a single digital optical signal at output port 755.

According to this embodiment of the present invention, the bit stream generated by the interleaver 745 is clocked by an external clock signal, rather than by an internally generated clock signal. The external clock signal is received at clock port 760 and provided to a clock recovery circuit 732 which can comprise, for example, a bandpass filter. The output of the clock recovery circuit 732 is coupled to the interleaver 745 for clocking the interleaver 745 at the clock speed or some multiple thereof for transmission of frames of data. So that transmission speed is not compromised by the time-division-multiplexing of multiple reverse signals, the speed of the interleaved signal is preferably n times the speed of each reverse signal, where n is equal to the number of reverse signals received and interleaved by the digital optical transmitter 800.

As mentioned above, FIG. 8 is an illustration of a digital optical transmitter 800 included in an optical node 715. One of ordinary skill in the art will understand that a transmitter 800 included in the hub 705 is similar, but the A/D converters 725 and serializers 730 would be replaced with optical detectors, such as detector 310 (FIG. 6), for generating a digital electrical signals that could be provided directly to the interleaver 745.

It will further be understood that, when a single optical signal is received, such as by the headend 105, the transmitting device (e.g., the hub 705) and the receiving device (e.g., the headend 105) need not necessarily receive the same external clock signal. Instead, since there is no interleaving of the received signal with which to contend, the transmitting device could mix a digital pilot tone with the information signal, as described in FIG. 2. Alternatively, synchronization information could be transmitted by the transmitting device and a bit synchronizer employed by the receiving device so that the same clock signal does not need to be used and so that a pilot tone does not have to be transmitted. However, as mentioned above, use of a synchronizing clock signal is desirable for situations in which a device (e.g., the hub 705) receives multiple signals that must be interleaved for retransmission as a single signal.

FIG. 9 shows an electrical block diagram of an optical transmitter 810 including a laser module 865 and a digital module 860. According to the present invention, the digital module 860 includes at least one A/D converter 725 for converting an incoming, upstream analog electrical signal, received at input port 720, to a digital electrical signal and at least one serializer 730 for converting the digital electrical signal into a serial bit stream. If the digital optical transmitter 810 is to process more than one upstream signal when it is installed in the cable television system, the digital module 860 can include multiple input ports 720, multiple A/D converters 725, multiple serializers 730, and an interleaver 745 for interleaving the incoming serial bit streams. The digital optical transmitter 810, as shown, includes circuitry within the digital module 860 for processing two upstream electrical signals, although it will be appreciated that the number of input ports 720 and corresponding processing paths will be determined by system needs in the reverse path.

The transmitting circuitry, such as the optical driver 748 and the laser diode 750, for transmitting the interleaved bits of data can, for example, be included in a laser module 865. Clock circuitry, such as the clock input port 760, clock recovery circuit 732, A/D converter 735, and controller 740, can be separate from the laser module 865 and digital module 860 or could alternatively be included in either module 860, 865.

According to the present invention, the digital optical transmitter 810 can be upgraded by installing additional or replacement digital modules, as shown in FIG. 10. In the digital optical transmitter 810 of FIG. 10, an additional two-port digital module 890 has been inserted into the digital transmitter 810. The additional module 890 also includes one or more additional input ports 820, one or more A/D converters 825, one or more serializers 830, and, if necessary, an interleaver 845. Additionally, at some time before the transmitter 810 is upgraded to process other upstream paths, or during such upgrade, a multiplexer 850 can installed within the transmitter 810 to multiplex the interleaver outputs and thereby provide a single serial bit stream to the laser module 865. Preferably, the digital modules 860, 890 are each housed in a mechanical unit that can be removed from and inserted into a digital optical transmitter 810 to upgrade or downgrade the transmitter 810 as needed. A digital module 890 could, for instance, snap into a transmitter 810 in such a manner that the input ports 820 mate with openings in the housing (not shown) of the transmitter 810. At the same time, electrical terminals (not shown) at the output of the interleaver 845 and at the clock input of the interleaver 845 could, merely by insertion, be physically and electrically coupled, respectively, to a multiplexer input and the output of the controller 740 or other clock circuitry. Alternatively, the interleaver output and the clock input of the interleaver 845 could be manually coupled, such as by soldering or riveting, to the appropriate electrical devices after insertion of the digital module 890 into the digital optical transmitter 810.

If it is anticipated that all upgrades to the transmitter 810 will require the insertion of additional digital modules 890, one of ordinary skill in the art will recognize that the initially installed reverse path circuitry need not be housed in a removable mechanical unit that is able to be inserted and removed from the transmitter 810. Instead, the initial reverse path circuit (e.g., A/D converter, serializer, and interleaver) could be stationary and fixed within the transmitter 810 as long as the transmitter 810 can physically and electrically accommodate additional digital modules 890 that may be required to upgrade the transmitter 810.

Alternatively, all digital modules 860, 890 for generating digital bit streams could be manufactured as electrically and mechanically detachable units so that each unit can be removed. In such an embodiment of the present invention, even the initially installed reverse path circuit could be replaced so that, by way of example, an initial two-port digital module could be later replaced with a four-port, six-port, or eight-port digital module while the transmitter 810 is in the field. According to an alternative embodiment of the present invention, each digital module 860,

890 could include its own dedicated multiplexer 850. In this situation, a digital module 860 having, for instance, two inputs could be replaced with a different digital module (not shown) have a greater number of inputs or fewer inputs, depending upon system needs. According to this alternative embodiment, the transmitter 810 would only have to have space to accommodate a single digital module, since the initial digital module would be replaced to upgrade the system, rather than adding additional digital modules to upgrade the system.

As described above, the digital optical transmitter 810 can be easily upgraded with minimal expense, time, and labor to transmit additional return paths on a single fiber optic cable. Furthermore, by providing mechanically separate and/or replaceable digital modules as described above, additional reverse paths can be included without having to purchase and install additional optical transmission device, e.g., the laser diode 750 and driver 748, and these parts are usually the most expensive circuitry of the digital optical transmitter 810.

Claims

What is claimed is:CLAIMS

1. An optical transmitter for receiving electrical signals that are transmitted in a reverse direction and for transmitting optical signals, corresponding to the electrical signals, in the reverse direction, comprising: a reverse path circuit for processing at least one analog electrical signal to generate therefrom a first digital bit stream; a digital module that is detachably mounted within the optical transmitter for processing additional analog electrical signals to generate therefrom a second digital bit stream, wherein the digital module comprises a mechanical unit that can be mechanically and electrically inserted into and removed from the optical transmitter; a multiplexer coupled to the reverse path circuit and the digital module for receiving the first and second digital bit streams and providing a multiplexed output; and a laser module coupled to the multiplexer for receiving the multiplexed output and generating therefrom a digital optical signal that is transmitted by the optical transmitter in the reverse direction.

2. The optical transmitter of claim 1, wherein the multiplexer includes a plurality of inputs for receiving outputs of additional digital modules that can be added to the optical transmitter to provide additional reverse path capability.

3. The optical transmitter of claim 2, wherein the laser module comprises: a single laser diode for emitting the digital optical signal; and a single optical driver coupled to the single laser diode for receiving the multiplexed output generated from the reverse path circuit, the digital module, and any other removable digital modules such that the digital optical signal can be transmitted over a single fiber optic cable.

4. The optical transmitter of claim 3, wherein the digital module comprises: an input port for receiving an additional analog electrical signal; an analog-to-digital converter for converting the additional analog electrical signal into a digital electrical signal; and a serializer for converting the digital electrical signal into the second digital bit stream.

5. The optical transmitter of claim 3, wherein the digital optical module comprises: first and second input ports for receiving first and second additional analog electrical signals; first and second analog-to-digital converters for converting the first and second additional analog electrical signals to first and second digital electrical signals; first and second serializers for converting the first and second digital electrical signals to first and second bit streams; and an interleaver coupled to the first and second serializers for interleaving the first and second bit streams to generate the second digital bit stream.

6. A cable television system for processing cable television signals, the cable television system comprising: headend equipment for transmitting forward digital optical signals and for receiving reverse digital optical signals; subscriber equipment for receiving forward analog electrical signals and for transmitting reverse analog electrical signals; an optical node coupled between the headend equipment and the subscriber equipment for converting the forward digital optical signals into forward analog electrical signals for continued transmission through the cable television system to the subscriber equipment and for converting the reverse analog electrical signals into the reverse digital optical signals, wherein the optical node includes an optical transmitter in a reverse path of the optical node, the optical transmitter comprising: a reverse path circuit for processing at least one reverse analog electrical signal to generate therefrom a first digital bit stream; a digital module that is detachably mounted within the optical transmitter for processing additional reverse analog electrical signals to generate therefrom a second digital bit stream, wherein the digital module comprises a mechanical unit that can be mechanically and electrically inserted into and removed from the optical transmitter; a multiplexer coupled to the reverse path circuit and the digital module for receiving the first and second digital bit streams and providing a multiplexed output; and a laser module coupled to the multiplexer for receiving the multiplexed output and generating therefrom a reverse digital optical signal that is transmitted by the optical transmitter in the reverse direction.

7. The cable television system of claim 6, further comprising: a first communication medium coupled between the headend equipment and the optical node; and a second communication medium coupled between the optical node and the subscriber equipment.

8. The cable television system of claim 7, wherein the multiplexer of the optical transmitter includes a plurality of inputs for receiving outputs of additional digital modules that can be added to the optical transmitter to provide additional reverse path capability.

9. The cable television system of claim 8, wherein the laser module of the optical transmitter comprises: a single laser diode for emitting the reverse digital optical signal; and a single optical driver coupled to the single laser diode for receiving the multiplexed output generated from the reverse path circuit, the digital module, and any other removable digital modules such that the reverse digital optical signal can be transmitted over a single fiber optic cable that comprises the first communication medium.

10. The cable television system of claim 9, wherein the digital module of the optical transmitter comprises: an input port for receiving an additional reverse analog electrical signal; an analog-to-digital converter for converting the additional reverse analog electrical signal into a digital electrical signal; and a serializer for converting the digital electrical signal into the second digital bit stream.

1 1. The cable television system of claim 9, wherein the digital optical module comprises: first and second input ports for receiving first and second additional analog electrical signals; first and second analog-to-digital converters for converting the first and second additional analog electrical signals to first and second digital electrical signals; first and second serializers for converting the first and second digital electrical signals to first and second bit streams; and an interleaver coupled to the first and second serializers for interleaving the first and second bit streams to generate the second digital bit stream.

12. An optical transmitter for receiving electrical signals that are transmitted in a reverse direction and for transmitting optical signals, corresponding to the electrical signals, in the reverse direction, comprising: a reverse path circuit for processing at least one analog electrical signal to generate therefrom a first digital bit stream; a digital module that is detachably mounted within the optical transmitter for processing additional analog electrical signals to generate therefrom a second digital bit stream, wherein the digital module comprises a mechanical unit that can be mechanically and electrically inserted into and removed from the optical transmitter; a multiplexer included within the digital module for receiving the first and second digital bit streams and providing a multiplexed output; a laser module coupled to the multiplexer for receiving the multiplexed output and generating therefrom a digital optical signal that is transmitted by the optical transmitter in the reverse direction, wherein the digital module can be replaced with a different digital module, including its own multiplexer, that generates an additional number of digital bit streams from which to provide the multiplexed output.

13. The optical transmitter of claim 12, wherein the laser module comprises: a single laser diode for emitting the digital optical signal; and a single optical driver coupled to the single laser diode for receiving the multiplexed output generated from the reverse path circuit, the digital module, and any other removable digital modules such that the digital optical signal can be transmitted over a single fiber optic cable.

14. The optical transmitter of claim 13, wherein the digital module further comprises: an input port for receiving an additional analog electrical signal; an analog-to-digital converter for converting the additional analog electrical signal into a digital electrical signal; and a serializer for converting the digital electrical signal into the second digital bit stream.